In wireless communication using mobile phones for example, a transmitter for transmitting data may modulate transmission data by quadrature modulation system such as QPSK, QAM, and the like to be transmitted to a receiver. In this case, fluctuation in the amplitude value (amplitude shift) and the like is generated in the data received by the receiver at the time of modulation by the transmitter. There is a problem that when the receiver demodulates a modulated signal with amplitude shift and the like being generated, characteristics of the reception signal deteriorate.
In order to solve such a problem, Patent Literature 1 discloses a quadrature modulation device that demodulates a signal generated by a quadrature modulator using a quadrature demodulator in the same device, and corrects the gain and the like of a signal to be quadrature modulated by the quadrature modulator using the demodulated signal. The quadrature modulation device disclosed in Patent Literature 1 is explained using FIG. 5.
The quadrature modulation device includes a signal for modulation generator 101, a correction unit 102, a digital-to-analog (D/A) converter 103, a quadrature modulator 104, a switch 105, a first local signal generator 106, a second local signal generator 107, a control unit 108, a frequency converter 109, an analog-to-digital (A/D) converter 110, a quadrature modulator 111, an error calculation unit 112, and an output terminal 113.
The quadrature modulator 104 generates a quadrature modulated signal X with a carrier frequency fc using an in-phase component signal I, a quadrature component signal Q, and a first local signal La that is output from the first local signal generator 106. The quadrature modulated signal X is output to the output terminal 113 through the switch 105 in a normal mode. Moreover, the quadrature modulated signal X is output to the frequency converter 109 through the switch 105 in a calibration mode.
The frequency converter 109 mixes the input quadrature modulated signal X and a second local signal with a frequency (fc-fi) that is output from the second local signal generator 107, and extracts only a signal Y in an intermediate frequency band centered on the frequency fi. The A/D converter 110 converts the signal Y output from the frequency converter 109 into a digital signal and outputs it to the quadrature demodulator 111.
The quadrature demodulator 111 multiplies a local signal with phases in quadrature and the frequency fi by the signal Y so as to demodulate the in-phase component signal I and the quadrature component signal Q in a modulation frequency band that is sufficiently lower than the intermediate frequency. The error calculation unit 112 calculates an amplitude error, a phase error, and the like in the quadrature modulator 104 based on the in-phase component signal I and the quadrature component signal Q. The control unit 108 notifies the correction unit 102 of a correction value that offsets the amplitude error, phase error, and the like that are calculated by the error calculation unit 112. The correction unit 102 executes correction on the in-phase component signal I and the quadrature component signal Q output from the signal for modulation generator 101 using the correction value notified by the control unit 108, and outputs them to the D/A converter 103.
Then, the quadrature modulation device can output a signal with corrected amplitude error, phase error, and the like that are generated in the quadrature modulator 104.